ANALYTICAL EDITION
December, 1946
801
which was read with a cathetometer to *0.01 mm. The sensitivity may be expressed as the change in heater current for a 1% change in pressure. This was determined by graphical differentiation. When the sensitivity is plotted against the logarithm of the pressure, it falls off symmetrically from a maximum of 0.31 ma. a t 0.2 mm., reaching 0.10 ma. a t 0.02 and 2 mm., and 0.01 ma. a t 0.002 and 20 mm. I n any one run the current could be determined to *0.01 ma. This means that the gage is sensitive to a 1% change in pressure from 0.002 to 20 mm., while from 0.02 to 2 mm. it is sensitive to 0.170 or less. From one run to another, however, readings varied by as much as 3 ma, This was attributed to external temperature changes. By attaching a thermocouple to the outside of the gage, it was found that a change of 1" C. in its surface temperature changed the reading about 1 ma. The gage was therefore enclosed in a metal jacket, through which was circulated constant-temperature water. Although the surface temperature was slightly dependent on pressure, it was constant within O.0lo C. a t any one pressure.
in the tube, measured by a three-junction copper-constantan thermopile, was thus kept constant t o *0.02" C., which determines the pressure within 0.27", the precision of the published data. As much as 1hour may be required to reach equilibrium. The fluctuations in the gage reading are slightly larger than with air about 0.02 ma. This may be due to thermal fluctuations, wdich might be expected to amount to as much as 0.06 ma. in the most sensitive region, but are largely smoothed out by the s l o ~ response.
The major objection to any thermal pressure gage is the curved characteristic which necessitates extensive calibration against some standard. The McLeod gage is not accurate enough to utilize the full sensitivity of the thermocouple gage. Since it appears that a principal use of the gage as a precise instrument would be in measuring the pressure of water vapor above hydrates, a convenient standard is the vapor pressure of ice, mhich IS known ( 2 ) to *0.27,. Although a complete calibration curve has not been made, enough points have been measured to show that this is feasible.
There seems to be no reason why a Pirani gage operated a t constant temperature and with sufficiently accurate electrical instruments should not be as precise as the thermocouple gage. However, a metal filament should be used. The author has found that carbon filament bulbs, recommended by De Vries ( I ) , are excessively difficult to outgas. Although such a gage is sensitive to i0.170 in the range 0.1 to 0.5 mm., duplicate runs on water vapor agreed only to 1%'no doubt because of residual air.
The ice was contained in a tube lagged by a n oil jacket and immersed in an alcohol bath whose temperature was controlled by a resistance-thermometer type of regulator, operating a pump which circulated alcohol from a dry-ice bath. The temperature
(1) De Vries, T., J . OpticaE SOC.Am., 18,333 (1929). (2) International Critical Tables, 1'01. 3, 210, New York, McGrawHill Book Co., 1926. (3) Rittner, Rev. Sci. Instruments, 17, 113 (1946).
Determination of
Gold
The followng readings, made in succession, show the reproducibility of the results whether equilibrium is approached from above or below: Temperature,
'C
Pressure, Mm.
0 0497
-45 -36
73 69 - 2 3 a4 -36 69
0 1399 0 529 0 1399
Current, M a . 60 98 f 0 01 86 42 f 0 02 115 11 * O 02 86 42 f 0 02
LITERATURE CITED
with Ascorbic A c i d
E. C. STATHIS AND H. C. GATOS, Inorganic Chemistry Laboratory, Athens University, Athens, Greece
SCORBIC acid or vitamin c Tvasisolated by Szent-Gyorgyi (2) and and its synthesis was later by
A (dl
Haworth and Co-wOrkers ( 2 ) . Haworth and Hirst assigned the accepted structural formula which includes the endiol group. - The reducing property of this group is shown in the reduction of gold chloride as represented by the follo\ving equation: 2.1UCIj f 3CbH806 +2Au
+ 3C~H606f
6HC1
The detailed investigation of this reaction in the authors' laboratory (3) proved that ascorbic acid could be used for the quantitative determination of gold. The present report describes the procedure recommended. REAGFXTSUSED. Hydrochloric acid, density 1.19. Ascorbic acid, strength, 4 grams per 100 ml. of solution. Gold solutions, prepared from pure gold as determined by sulfur dioxide method. PROCEDURE. Dissolve 0.3 gram or less of gold in aqua regia and remove the nitric acid, nitrites, etc., by three evaporations with hydrochloric acid. Dissolve the residue in 3 to 5 ml. of concentrated hydrochloric acid and dilute to 20 ml. with water.
Table Gold Added Gram 0.2060 0.1585 0.0969 0.0152 0.2030 0.1215 0.0949 0.0678 0.140$ 0.1051 0.0947 0.0653
I.
Gold Found
Gram
Gram
... ... ... o .'idso
0.2061 0.1586 0.0968 0.0152 0.2032 0.1217 0,0948 0.0580
0.1208 0,1930 0.0281 . . I
The results are included in Table I. The influence of copper was investigated and the results in Table I prove that there is no interference. VOLUMETRIC DETERMINATION
Gold may be determined volumetrically with ascorbic acid by adding excess reagent to the cold solution of gold and subsequently titrating the excess with iodine. The reaction of ascorbic acid and iodine is described by t h e following equation: CeH8O6
+ 1 2 +C6H6O6 + 2HI
PROCEDURE. Free the gold solution from oxidizing reagents, add 2 to 5 ml. of hydrochloric acid reagent, and dilute with water td 20 ml. .4dd 10 ml. of ascorbic acid solution with constant stirring for 5 minutes, then 2 ml. of starch solution, and titrate with 0.1 X iodine solution. Blank determinations are required. Some of the results obtained are recorded in Table I.
Determination of Gold
Copper Added
... ... ...
Heat the solution to 80"to 90" C., add 10 ml. of the freshly prepared ascorbic acid reagent, and continue the heating for 5 n i n utes. After cooling to room temperature, filter the precipitate by a porcelain filter crucible, Xvash with hydrochloric acid solution, 1to 99, and ignite. -
CONCLUSION
Method
Gravimetric
Gold may be determined with excellent accuracy by reduction of gold chloride with ascorbic acid. Copper does not interfere. Gold can also be determined volumetrically with ascorbic acid. LITERATURE CITED
(1) Haworth, N. IT., J . Chem. Soc., 1933, 1370. (2) Reichstein, T., Helv. Chim. Acta, 16, 1020 (1933). (3) Stathis, C. E., Praktika Akad. Athenon, Communication 16
(April, 1942). (4) Szent-Gyorgyi, Biochem. J., 22, 1387 (1928).
